1. Field of the Invention
This invention relates generally to catalytic hydrogen transfer reactions involving the dehydrogenation of paraffins or naphthenes or the dehydrocyclization of paraffins and more particularly concerns improvements in such reactions based upon the use of an improved metal-containing active carbon catalyst.
2. Description of the Prior Art
Catalytic hydrogen transfer reactions involving the dehydrogenation of paraffins or naphthenes or the dehydrocyclization of paraffins occur in numerous refinery and chemical processing operations. For example, such dehydrogenation and dehydrocyclization reactions are important reactions in the catalytic reforming of hydrocarbon streams in refinery operations. It is highly desirable to maximize the selectivity and efficiency with which such hydrogen transfer reactions are performed.
It is known that the presence of metals in active carbon can greatly enhance the efficiency and selectivity of the active carbon when it is employed in catalytic, sorption, or filtering applications. However, attempts to incorporate metal compounds into activated carbon by conventional physical impregnation techniques have been problematical. One disadvantage with physical impregnation of activated carbon with metal compounds is that the small pores at the surface of the active carbon particles are inaccessible to liquid penetration and prevent penetration of the liquid, metal-containing impregnating solutions, thereby rendering impossible uniform and thorough impregnation of the carbon particles with metal. Furthermore, physical impregnation of the active carbon causes partial blocking of the pores of the carbon particles resulting in an appreciable reduction of the active surface area thereof. In addition, it is not possible to control to any large extent the total quantity of the metal applied to the active carbon particles by impregnation and its distribution on and in the carbon particles, with the end result that there is a substantial risk that the metal will crystallize and agglomerate in an undesirable manner on the carbon particles.
Several techniques have been proposed to overcome the problems associated with impregnating active carbon with metal compounds. For example, Dimitry, U.S. Pat. No. 3,886,093 discloses activated carbons having uniformly distributed active metal sites and a method for making such activated carbons. The method of Dimitry involves mixing an aqueous solution of a lignin salt with an aqueous solution of a transition metal salt to precipitate the transition metal and lignin as a metal lignate. The transition metal must be capable of forming a chemical bond with the lignin and in so doing precipitating the lignin from solution as a metal lignate. Dimitry discloses that the time required to complete the precipitation is less than one hour and that usually 30 minutes is sufficient for this purpose. According to Dimitry, suitably the wet metal lignate precipitate can then be dried in a spray drier. Dimitry states that, although drying the metal lignate precipitate is not critical to form an activated carbon product, drying is necessary to form a high surface area end product. However, Dimitry gives neither a general disclosure nor a specific example of what it means by a "high surface" area for its end product. Dimitry states that the active metal sites are uniformly distributed throughout the activated carbon end product and presents an electron micrograph of an activated carbon end product magnified 5,700 times. However, from this relatively low magnification micrograph, the distribution of the active metal sites in the activated carbon end product is not readily apparent.
Furthermore, Siren, U.S. Pat. No. 4,242,226 states that the metal content in the active carbon which can be achieved by pyrolysis and activation of a metal lignate precipitate is much too low for the majority of fields of use and that it is difficult using such technique to predetermine the properties of the resulting metal-containing active carbon end product owing to the substantially undefined structure of the lignin. Siren discloses an alternative technique in which a cation of calcium, magnesium, barium, aluminum, copper or a transition metal and an anionic group chemically bound to a polyhexose derivative are caused to react in solution, and the resulting product is precipitated either spontaneously or by adding a suitable precipitating agent. Siren discloses that, after separating the precipitate from solution, the precipitate can, if desired, be dried, for example, by spray drying. Thereafter the separated reaction product is pyrolyzed and activated to form the activated carbon. In the method of Siren, suitably the polyhexose derivative employed comprises an acid polyhexose derivative and preferably the anionic groups of the polyhexose derivative comprise carboxylic acid groups, sulfonic acid groups or phosphoric acid groups. Preferably the polyhexose derivatives contain from 1 to 3 metal cations per hexose unit.
Wennerberg et al., U.S. Pat. No. 4,082,694 disclose a process for making a high surface area active carbon by first heating an agitated combination of solid potassium hydroxide containing between 2 and 25 weight percent water and a carbonaceous material comprising coal coke, petroleum coke or a mixture thereof below about 483.degree. C., then heating the resulting dehydrated product at a temperature between 705.degree. C. and 983.degree. C. to thereby form active carbon, and finally cooling the resulting activated product and removing essentially all of the inorganic material therefrom by water washing to form the high surface area active carbon end product. The resulting product is a high surface area active carbon material which has a cage-like structure exhibiting a microporosity which contributes to over 60 percent of its surface and which has an effective BET surface area of greater than about 2500 square meters per gram and a bulk density greater than about 0.25 gram per cubic centimeter. Wennerberg et al., U.S. Pat. Nos. 3,642,657 and 3,817,874 and Wennerberg, U.S. Pat. No. 3,726,808 disclose related methods for making high surface area active carbon products.
More recently, in my copending patent application Ser. No. 470,285 and copending patent application Ser. No. 470,487, both filed on Feb. 28, 1983, and both of which in their entirety are specifically incorporated herein by reference, I disclosed an active carbon composition comprising a substantially uniform dispersion of a metal or metal-containing material in a porous carbon matrix, wherein the weight ratio of the metal or metal-containing material to active carbon matrix is from about 1:10,000 to about 1:1, based on the weight of the metal or metal-containing material, respectively, and having a cage-like structure and a BET surface area of at least 800 square meters per gram and a bulk density of at least 0.1 gram per cubic centimeter, and two suitable methods for preparing such metal-containing active carbon compositions.
The preparation disclosed in my copending patent application Ser. No. 470,285 comprises the following steps: forming a uniform co-crystallite of a precursor of the metal or metal-containing material and of a carbon precursor, wherein the metal in the precursor of the metal or of the metal-containing material is a transition metal or a metal from Groups IIIA, IVA or VA of the Periodic Table of the Elements; forming a uniform powdered mixture of the co-crystallite and organic solids comprising an alkali metal hydroxide; pyrolizing the powdered mixture in an inert atmosphere at a temperature in the range of from about 400.degree. C. to about 980.degree. C. to form the carbon matrix having the metal or metal-containing material substantially uniformly dispersed therein; and separating unreacted inorganic material and inorganic reaction products other than the dispersed metal or metal-containing material from the porous carbon matrix.
The method disclosed in my copending application Ser. No. 470,487 comprises the following steps: forming a carbon precursor which contains the metal by the chemical reaction in solution of (1) a soluble carbon precursor having at least one anionic group chemically bound thereto and (2) a soluble cation of a transition metal or metal from Groups IIIA, IVA or VA of the Periodic Table or a soluble cationic complex of such metal cation; precipitating and drying the metal-containing carbon precursor; forming a uniform powdered mixture of the metal-containing carbon precursor and inorganic solids comprising an alkali metal hydroxide; pyrolyzing the powdered mixture in an inert atmosphere at a temperature in the range of from about 400.degree. C. to about 980.degree. C. to form the carbon matrix having the metal or metal-containing material substantially uniformly dispersed therein; and separating unreacted inorganic material and inorganic reaction products, other than the dispersed metal or metal-containing material, from the carbon matrix to form the high surface area, porous carbon matrix end product.